Add a triple_buffer class that allows publishing data in a wait-free manner from a real-time thread to another thread.

It's largely inspired from the crate cubeb-coreaudio-rs uses, but ported to
C++.

CMakeLists.txt

@@ -400,6 +400,7 @@ if(BUILD_TESTS)
 
   cubeb_add_test(duplex)
   cubeb_add_test(logging)
+  cubeb_add_test(triple_buffer)
 
   if (USE_WASAPI)
     cubeb_add_test(overload_callback)

src/cubeb_triple_buffer.h

@@ -0,0 +1,80 @@
+/*
+ * Copyright © 2022 Mozilla Foundation
+ *
+ * This program is made available under an ISC-style license.  See the
+ * accompanying file LICENSE for details.
+ */
+
+/**
+ * Adapted and ported to C++ from https://crates.io/crates/triple_buffer
+ */
+
+#ifndef CUBEB_TRIPLE_BUFFER
+#define CUBEB_TRIPLE_BUFFER
+
+#include <atomic>
+
+// Single producer / single consumer wait-free triple buffering
+// implementation, for when a producer wants to publish data to a consumer
+// without blocking, but when a queue is wastefull, because it's OK for the
+// consumer to miss data updates.
+template <typename T> class triple_buffer {
+public:
+  // Write a new value into the triple buffer. Returns true if a value was
+  // overwritten.
+  // Producer-side only.
+  bool write(T & input)
+  {
+    storage[input_idx] = input;
+    return publish();
+  }
+  // Get the latest value from the triple buffer.
+  // Consumer-side only.
+  T & read()
+  {
+    update();
+    return storage[output_idx];
+  }
+  // Returns true if a new value has been published by the consumer without
+  // having been consumed yet.
+  // Consumer-side only.
+  bool updated()
+  {
+    return (shared_state.load(std::memory_order_relaxed) & BACK_DIRTY_BIT) != 0;
+  }
+
+private:
+  // Publish a value to the consumer. Returns true if the data was overwritten
+  // without having been read.
+  bool publish()
+  {
+    auto former_back_idx = shared_state.exchange(input_idx | BACK_DIRTY_BIT,
+                                                 std::memory_order_acq_rel);
+    input_idx = former_back_idx & BACK_INDEX_MASK;
+    return (former_back_idx & BACK_DIRTY_BIT) != 0;
+  }
+  // Get a new value from the producer, if a new value has been produced.
+  bool update()
+  {
+    bool was_updated = updated();
+    if (was_updated) {
+      auto former_back_idx =
+          shared_state.exchange(output_idx, std::memory_order_acq_rel);
+      output_idx = former_back_idx & BACK_INDEX_MASK;
+    }
+    return was_updated;
+  }
+  T storage[3];
+  // Mask used to extract back-buffer index
+  const uint8_t BACK_INDEX_MASK = 0b11;
+  // Bit set by producer to signal updates
+  const uint8_t BACK_DIRTY_BIT = 0b100;
+  // Shared state: a dirty bit, and an index.
+  std::atomic<uint8_t> shared_state = {0};
+  // Output index, private to the consumer.
+  uint8_t output_idx = 1;
+  // Input index, private to the producer.
+  uint8_t input_idx = 2;
+};
+
+#endif // CUBEB_TRIPLE_BUFFER

test/test_triple_buffer.cpp

@@ -0,0 +1,67 @@
+/*
+ * Copyright © 2022 Mozilla Foundation
+ *
+ * This program is made available under an ISC-style license.  See the
+ * accompanying file LICENSE for details.
+ */
+
+/* cubeb_triple_buffer test  */
+#include "gtest/gtest.h"
+#if !defined(_XOPEN_SOURCE)
+#define _XOPEN_SOURCE 600
+#endif
+#include "cubeb/cubeb.h"
+#include "cubeb_triple_buffer.h"
+#include <atomic>
+#include <math.h>
+#include <memory>
+#include <stdio.h>
+#include <stdlib.h>
+#include <thread>
+
+#include "common.h"
+
+TEST(cubeb, triple_buffer)
+{
+  struct AB {
+    uint64_t a;
+    uint64_t b;
+  };
+  triple_buffer<AB> buffer;
+
+  std::atomic<bool> finished = {false};
+
+  ASSERT_TRUE(!buffer.updated());
+
+  auto t = std::thread([&finished, &buffer] {
+    AB ab;
+    ab.a = 0;
+    ab.b = UINT64_MAX;
+    uint64_t counter = 0;
+    do {
+      buffer.write(ab);
+      ab.a++;
+      ab.b--;
+    } while (counter++ < 1e6 && ab.a <= UINT64_MAX && ab.b != 0);
+    finished.store(true);
+  });
+
+  AB ab;
+  AB old_ab;
+  old_ab.a = 0;
+  old_ab.b = UINT64_MAX;
+
+  // Wait to have at least one value produced.
+  while (!buffer.updated()) {
+  }
+
+  // Check that the values are increasing (resp. descreasing) monotonically.
+  while (!finished) {
+    ab = buffer.read();
+    ASSERT_GE(ab.a, old_ab.a);
+    ASSERT_LE(ab.b, old_ab.b);
+    old_ab = ab;
+  }
+
+  t.join();
+}